1. Field of the invention
The present invention relates to a shift actuator which helps perform electrical gear change of a propeller shaft for an outboard motor mounted on a vessel.
2. Description of the Related Art
There are known various kinds of shift actuators, each being included in an outboard motor and electrically actuating a gearshift for a propeller shaft. For example, there is a shift actuator which causes an electric motor to rotate a shift rod and activates replacement of gears between an ahead gear and an astern gear.
In such a shift actuator, an electric motor rotates a shift rod. The rotation of the shift rod is changed into a rectilinear motion which moves coaxially with a propeller shaft. The rectilinear motion is transmitted to a shift slider. The shift slider slides along the propeller shaft. As the shift slider slides, it moves a dog clutch which has three positions, one being an ahead position, another a neutral position, and the third an astern position. The dog clutch is made to move to one of the three positions by the shift slider, and, when the dog clutch is brought to either the ahead position or the astern position, it is suitably joined to either one of the ahead gear and astern gear which are provided in the propeller shaft, thereby accomplishing a change of gears. In this way, shifting a gear using an electric motor will improve a gear change response, resulting in achievement of a quick gear change.
It should be noted that an electric motor that rotates a shift rod should be arranged near a shift slider or a dog clutch (in short, a propeller shaft) to make the shift rod slide the shift slider. This is because not only reduction in the overall length of a shift rod will be achieved but also it will be easy to promote transmission efficiency of motor power, improvement in transmission accuracy of motor power, and miniaturization of a shift actuator. Therefore, the shift actuator which includes the electric motor is arranged in the gearshift case which is located in a lower part (lower unit) which the outboard motor has.
However, the shift slider in the shift actuator must axially slide along the propeller shaft in order to move the dog clutch. Therefore, a movable region (space) which extends along the propeller shaft should be secured in the gearshift case.
It should be noted here that a gearshift case generally contains many accommodation parts inside of it. Therefore, there is a possibility that a sufficiently large movable region, which extends along a propeller shaft, cannot be secured in a shift actuator. Moreover, a shift actuator becomes long along a propeller shaft by the length of a movable region. Therefore, there is a possibility that miniaturization of a shift actuator cannot be sufficiently achieved.